Amplifiers are used to increase the voltage and current levels of a signal in order to drive a load. All amplifiers use a power supply that supplies the energy required to perform this task. The power supply converts the AC line voltage into DC voltages suitable for the amplifier. The power supply also isolates the line voltage from the DC output voltages for safety purposes. This is accomplished by the transformer. A conventional power supply in an amplifier includes a transformer that works directly off the 60 Hz line voltage, and tends to be relatively heavy and bulky. The output of many prior art AC to DC power supplies completely unregulated, with no current limiting. In many prior such power supplies, the input current typically has a power factor of less than 0.7, not meeting most international regulations.
To resolve these problems, the use of switching power supplies was implemented. Now there was both line and load regulation, and current limiting. Power factor was still less that 0.7, and generally they could not work over the universal input voltage range of 90-265 VAC without some sort of adjustment. The supplies were implemented with voltage fed topologies such as the half bridge topology.
To remedy the switching supply problems, a boost PFC (power factor correction) stage was added to the input of the power supply. The boost stage consists of a boost inductor coupled to a boost switch. The boost stage can be controlled to yield a high power factor. The resulting supply is fully regulated, overload protected, and power factor can exceed 0.95, with a fully universal input voltage range. Unfortunately, the addition of the PFC stage reduces the efficiency of the supply. Additional circuitry is necessary to limit inrush current. It also became apparent that in many cases with audio amplifiers that the fully regulated output voltage did not sound as good as the unregulated output of the conventional supply. This may be partly due to the use of large energy storage capacitors being located in the transformer primary side of the power supply, with only small filtering capacitors on the output or secondary side. This limits the instantaneous current available to the amplifier. The added stage also increases the parts count and complexity of the design. The present invention resolves these detriments associated with known switchmode supplies while maintaining their advantages.